1. Field of the Invention
The present invention is generally related to devices for determining the direction of a magnetic field and, more particularly, to a device which provides a digital output that is directly representative of a compass direction.
2. Description of the Prior Art
Many different ways of measuring magnetic fields are well known to those skilled in the art. For example, Hall effect devices and permalloy elements have been used in many different configurations to determine the magnitude or direction of a magnetic field.
U.S. Pat. No. 4,553,872, which issued to Boord et al on Aug. 6, 1985, discloses a magnetic field sensor element that is capable of measuring magnetic field components. The sensor can be rendered preferentially sensitive to magnetic field components in each of two, preferably orthogonal, directions by application of appropriate magnetic bias fields. When a first magnetic bias field is applied to the sensor element, the sensor is rendered sensitive to external magnetic field components in a first direction. When a second magnetic bias field is applied to the sensor element, the sensor is rendered sensitive to external magnetic field components in a second direction. The external magnetic field components measured by the sensor in this manner may then be used to calculate the orientation of the sensor with respect to the direction of the external magnetic field.
U.S. Pat. No. 4,503,394, which issued to Kawakami et al on Mar. 5, 1985, describes a magnetoresistive sensor which has a closed domain structure and electrode biasing. A magneto electric conversion element of this device comprises a magnetoresistive effect material having a closed domain structure, a pair of contacts for supply of current flowing through the magnetoresistive effect material and a bias electrode disposed between the contacts for biasing the direction of the current flow. The bias electrode is disposed so that the respective angles of intersection between the direction of current flow through different magnetic domains of the magnetoresistive effect material biased by the bias electrode and the directions of spontaneous magnetization biased by an external magnetic field are both increased or decreased.
U.S. Pat. No. 4,847,584, which issued to Pant on Jul. 11, 1989, discloses a magnetoresistive magnetic sensor which has a plurality of magnetoresistive material strips. The sensor has conductors positioned over the strips near the ends, but isolated therefrom. The strips are interconnected with interconnections located between the conductors. Strip ends may be tapered outside of the conductors. A plurality of arrangements of this type may be interconnected.
U.S. Pat. No. 5,199,178, which issued to Tong et al on Apr. 6, 1993, describes a thin film magnetic fluxgate compass having a supporting substrate on which a layer of high permeability material and at least two layers of nonmagnetic conducting materials are deposited. The high permeability material is fabricated to form a magnetic core, or cores, and a portion of the nonmagnetic conducting layers is fabricated to form an excitation coil which is connected to a pulse generator. The remainder of the nonmagnetic conducting layers is fabricated into at least two sensing coils or two pairs of sensing coils wound around the core in opposite pairs. A method for the fabrication of the thin film compass is also disclosed in this patent and the compass can be used for the determination of the direction of the geomagnetic field.
U.S. Pat. No. 5,175,936, which issued to Sato on Jan. 5, 1993, describes an electronic compass which can store azimuth data of points to be passed by a user in a plurality of registers. The stored azimuth data are sequentially displayed on a display device every time an external operation switch is operated. The compass includes a magnetic sensor for detecting a geomagnetism, thereby obtaining north azimuth data on the earth.
U.S. Pat. No. 5,161,311, which issued to Esmer et al on Nov. 10, 1992, discloses an electronic compass for use in a vehicle. The compass includes compensation for obtaining a high degree of accuracy without operator intervention or the need to drive the vehicle in a deliberate circular path. An automatic method for accurately determining maximum and minimum voltage values from a fluxgate sensor having orthogonal sensing windows is provided that operates continuously to adjust for required changes in both the offset and gain compensation factors.
U.S. Pat. No. 5,046,260, which issued to Wellhausen on Sep. 10, 1991, describes an electronic compass which has a plurality of sensors. Each sensor reacts to a component of the earth's field. It is desired to prevent errors based on variations in the signal amplification and to improve the accuracy of measurement. In accordance with this device, three identical sensor are disposed at an angle of 120 degrees and the compass may be used, in particular, for the navigation of vehicles.
U.S. Pat. No. 4,918,824, which issued to Farrar on Apr. 24, 1990, describes an electronic digital compass which utilizes an amorphous magneto resistive wire exhibiting reentrant behavior as a core. A sensing wire is wrapped around the core and the core is driven with a triangular wave through a drive coil. The earth's magnetic field biases the core such that when the core is driven to reentrant jumps the time duration between adjacent pulses induced into the sense winding is related to the heading. A pair of such arrangements are used in physical right angle relationship to resolve ambiguities in heading angle. The resulting heading angle calculation is a true digital product which is independent of analog amplitude variations and requires little or no adjustment for accuracy.
U.S. Pat. No. 4,640,016, which issued to Tanner et al on Feb. 3, 1987, discloses a remote indication compass which is formed by a magnetic compass and magnetoresistive sensors attached to the compass. It measures the field created by the magnetic compass. The signal of the sensors is analogous as well as cyclic with respect to the angle between the fastening plate of the sensors and the northern direction indicated by the magnetic compass. The length of a complete cycle is 360 degrees. The angle between the sensors is selected in such a manner that the flat areas of the signals do not coincide.
An article, entitled Magnetoresistive Sensors by Bharat B. Pant appeared in the Fall 1987 issue of the Scientific Honeyweller on pages 29-34. In this article, several different arrangements of magnetoresistive elements are described and several different applications of magnetoresistive magnetic detectors are discussed.
In the June 1982 issue of the Scientific Honeyweller three articles appeared which discuss magnetoresistive films. An article titled Properties of Thin Magnetoresistive Films, by George Wu, describes, on pages 34-36, the electrical and magnetic characteristics of magnetoresistive films. In addition to describing the anisotropy characteristic, both due to the material and the shape of a magnetoresistive film, this article also discloses empirical information relating to the relationship between the change in resistance of a magnetoresistive element and an external field applied to it. In the same issue of the Scientific Honeyweller, an article titled Thin Film Magnetic Sensors, by R. B. Fryer, describes, on pages 32-34, the magnetoresistive effect and certain applications of magnetic sensors. On pages 29-31 of the same issue of the Scientific Honeyweller, an article titled The Development of Thin-Film Sensors" by James Holmen, describes certain information obtained during the development of magnetic thin-films.
An article titled Magnetic Sensors, by James E. Lenz, appeared on pages 16-25 of the April 1985 issue of the Scientific Honeyweller. This article compares the operational ranges of several magnetic related technologies and describes experimental data obtained during the development of several of those technologies. In addition, this article discloses certain specific applications in which some of the magnetic sensing technologies have found particular application.
In a typical arrangement of magnetoresistive elements, the magnetically sensitive resistors are arranged in a Wheatstone bridge arrangement in order to measure the change in resistance which results from the effect of a magnetic field imposed on the resistors. By physically arranging four Wheatstone bridge resistors in an appropriate configuration, the effect on each bridge resistor can be used to provide an output signal that is representative of the angle between the Wheatstone bridge arrangement and the direction of a magnetic field. As the Wheatstone bridge arrangement is rotated relative to a magnetic field, the output signal from the Wheatstone bridge typically changes in a sinusoidal manner as a function of the angular relationship between the sensing axis of the Wheatstone bridge and the direction of magnetic field.
By using appropriate bias control devices, an analog output signal from a Wheatstone bridge of magnetoresistive elements can be used to determine the magnitude of a magnetic field component directed along the sensing axis of the magnetically sensitive Wheatstone bridge. A plurality of magnetic field component magnitudes, and their geometrical relationships, can then be used to determine magnetic field location. However, as is well known to those skilled in the art, the magnitude of the output signal is inherently nonlinear and consequently requires levels of precision in amplitude sensing and signal processing that are directly related to the required resolution and accuracy in determining the magnetic field direction.
Because of the nonlinearities in the output from a magnetically sensitive component comprising a Wheatstone bridge of magnetoresistive elements, it is well known to those skilled in the art that a precise determination of magnetic direction is difficult to accomplish. In other words, the change in magnetic field strength measured by a device of this type between a northerly direction and a northeasterly direction may not be equivalent to the change in magnetic field strength from a northeasterly direction to an easterly direction. Also, nonlinearities due to hysteretic material properties of magnetoresistive elements limit the repeatability of the analog signals produced by a Wheatstone bridge comprised of magnetoresistors and deleteriously affect determination of magnetic field direction. Care must be taken to account for these disadvantageous characteristics of magnetoresistive bridge magnetometers when the output signals of those devices are used in an analog manner.
It would therefore be significantly beneficial if a simplified technique is provided in a compass device for digitally decoding the direction of a magnetic field without the need to precisely measure the magnitude of the analog value of a magnetoresistive device.